A.c. electrolytic process

ABSTRACT

A PROCESS FOR PREPARING ORGANOMETALLIC COMPOUNDS OF THE FORMULA RYR&#39;&#39;N-YM WHERE R AND R&#39;&#39; ARE ALKYL, ALKENYL OR ARYL OF ABOUT 1-12 CARBONS AND MAY BE THE SAME OR DIFFERENT, M IS A METAL FROM GROUPS II-B, IV-A OR V-A, N CORRESPONDS TO THE VALENCE OF M AND Y IS A NUMBER FROM 0 TO 4, INCLUDING PASSING AN ELECTROLYZING ALTERNATING CURRENT THROUGH AN ELECTROLYTE SOLUTION BETWEEN ELECTRODES OF METAL M, THE SOLUTION CONTAINING A HYDROCARBYL GRIGNARD REAGENT RMGX, WHERE X IS A HALIDE OTHER THAN FLUORIDE, IN AN INERT ORAGNIC SOLVENT FOR THE REAGENT, THE SOLUTION FURTHER CONTAINING ABOUT 0.22.0 MOLES/MOLE OF RMGX OF A COMPOUND R&#39;&#39;X&#39;&#39;, WHERE X&#39;&#39; IS A HALIDE, OTHER THAN FLUORIDE OR LOWER ALKYL-SULFATE; AND RECOVERING RYR&#39;&#39;N-YM. USES OF SUCH PRODUCTS INCLUDE USE AS GASOLINE ANTIKNOCK COMPONENTS.   D R A W I N G

Dec. 28, 1911 J. B. GANCI ETAI- A.G. ELECTROLYTIC PROCESS Filed Aug. 24, 1970 vvvvvvvvb vvv vvvvvvvr United States Patent O 3,630,858 A.C. ELECTROLYTIC PROCESS .lames B. Ganci and Philip Manos, Wilmington, Del., assignors to E. I. du Pont de Nemours and Company,

Wilmington, Del.

Continuation-impart of application Ser. No. 774,765, Nov. 12, 1968. This application Aug. 24, 1970, Ser. No. 66,654

Int. Cl. Btllk 3/ 00 U.S. Cl. 204-59 18 Claims ABSTRACT OF THE DISCLOSURE A process for preparing organometallic compounds of the formula RyRn M where R and R' are alkyl, alkenyl or aryl of about 1-12 carbons and may be the same or different, M s a metal from Groups II-B, lV-A or V-A, n corresponds to the valence of M and y is a number from to 4, including passing an electrolyzing alternating current through an electrolyte solution between electrodes of metal M, the solution containing a hydrocarbyl Grignard reagent RMgX, where X is a halide other than uoride, in an inert organic solvent for the reagent, the solution further containing about 0.2- 2.0 moles/mole of RMgX of a compound RX, Where X is a halide, other than liuoride, or lower alkyl-sulfate; and recovering RyRn yM. Uses of such products include use as gasoline antiknock components.

This application is a continuation-inpart of application Ser. No. 774,765, iiled Nov. 12, 1968, now abandoned. The present invention relates to an improved electrolytic process for preparing hydrocarbyl metal compounds, in particular to utilizing alternating current for electrolysis of an electrolyte containing a Grignard reagent at metal electrodes which electrodes are composed of the metals desired in the hydrocarbyl metal compounds. The metals are, for example, lead, tin, bismuth and mercury. Such compounds are useful as gasoline antiknock components, stabilizers for resins, stabilizer intermediates, biocides, etc.

BACKGROUND It is well known to prepare tetrahydrocarbyl leads by passing direct current through an electrolyte consisting essentially of a Grignard reagent solution using a lead anode and an inert cathode. Lead is dissolved at the anode by oxidation and the dissolved lead reacts with the Grignard reagent to form a tetrahydrocarbyl lead. Magnesium metal is deposited at the cathode.

Practical utilization of this process is made diiiicult by two opposing factors. Minimal distance between anode and cathode is highly desirable to reduce the potential required to drive a practical electrolytic current through the Grignard reagent solution which has a relatively large electrical resistance. With close spacing, however, magnesium build-up at the cathode soon becomes sufficient to electrically short the electrodes and, thereby, to eiiectively stop desired electrolysis. Conversely, with widely spaced electrodes electrolyses over longer time periods are possible, but high potentials are required to give practical current through the electrolyte. As a result, heating can become excessive and heat dissipation a major problem.

It is known for direct current processes that magnesium build-up at the cathode may be reduced or eliminated by incorporating a hydrocarbyl halide coreagent in the Grignard reagent electrolyte solution. The halide dissolves the magnesium by reacting with it to form a Grignard reagent, which may be the same as or different, from the original Grignard reagent, depending on the coreagent 3,630,858 Patented Dec. 28, 1971 vCC used. lEven with the coreagent present, a serious drawback still remains. A conductive deposit forms at the anode and eventually shorts the electrodes. One way for overcoming this problem is to shut down the operation periodically while the direct current is reversed to redissolve the deposit at the anode, which becomes the cathode during .the shut-down. Tetrahydrocarbyl lead production is held SUMMARY OF THE INVENTION Process for preparing organometallic compounds of the formula RyRn yM where R and R are separately selected from alkyl and alkenyl gups of from 1 to about 8 carbons and aryl groups of from 6` to about 12 carbons, M is a metal selected from the metals of Groups II-B, IV-A and V-1A of the Periodic Table, n is a number corresponding to the valence of M and y is a number from 0 to 4, which includes:

(lA) passing an electrolyzing alternating electric current through an electrolyte solution between electrodes of a metal M as earlier defined, the electrolyte solution containing a hydrocarbyl Grignard reagent RMgX, where R is as above and X is a halide other than iluoride, in an inert organic solvent for the reagent, the solution also containing from about 0.2 to about 2.0 moles, per mole of RMgX of a compound R'X' where R' is as above defined and X is a halide other than fluoride or a lower alkylsulfate anion. eg. the methyl, ethyl, propyl or butylsulfate anion; and

(B) lRecovering the resultant RyR' 'M.

Preferred metals include lead, tin, bismuth and mercury, especially lead. R and R' are preferably methyl, ethyl, phenyl or vinyl, X is preferably chloride or bromide and X is preferably chloride, bromide or ethylsulfate. The concentration of the lGrignard reagent RMgX in the electrolyte solution is preferably about 0.5-3.0 molar. The preferred resulting products are the tetraalkyl leads. Other preferred embodiments are discussed in more detail below.

BRIEF DESCRIPTION OF DRAWING The accompanying drawing is a schematic diagram of a multielectrode cell with which the present invention may be practiced.

DETAILED DESCRIPTION OF 'INVENTION It has now been discovered that the above deficiencies of the direct current processes may be overcome by utilizing alternating current electricity as an electrolyzing current source. Alternating current electricity is operably connected electrically to alkylatable sacrificial metal electrodes. The electrodes are separted by an electrolyte consisting essentially of a hydrocarbyl Grignard reagent solution in a suitable organic solvent which solution contains a hydrocarbyl halide or a dihydrocarbyl sulfate. As the alternating current flow through the electrolyte is reversed in direction of ow with the normal cycling of the alternating current, each electrode becomes in turn anodic and then cathodic. When, during a cycle, an electrode is anodic, electrode metal is dissolved by oxidation at its surface and the dissolved metal reacts further with the Grignard reagent to form an organometallic compound. When, during the other half of the cycle, the electrode is cathodic, magnesium metal is deposited on its surface and continuously dissolved therefrom by formation of a Grignard reagent by reaction between the deposited magnesium and the hydrocarbyl halide in the solution, or a dissolved form of the magnesium from the reaction of magnesium with the dihydrocarbyl sulfate.

Two factors are requisite for the above alternating current process to operate, and to operate without significant short-circuit producing deposit formation:

(l) There must be a flow of electrolyzing alternating current through the electrolyte separating the electrodes. By electrolyzing current is meant sufficient current to provide desired electrochemical reaction at the electrodes. A minimum potential is required to provide such an electrolyzing current. The magnitude of the potential required to afford such a current varies widely with electrode metal, electrode area, the distance of separation of the electrodes, the electrical resistance of the electrolyte and the like. It is, therefore, impossible to assign a value to said minimum potential which value would be operable in all situations. However, it can be shown, for example, that if one assumes there is no electrical resistance in the electrolyte, an alternating current potential of about 0.5 volt (RMS) is required for detectable electrolytic reaction at lead electrodes of about 0.1 dm.2 surface area.

(2) The electrolyzing alternating current must be practically synmmetrical, i.e. have insufiicient D.C. bias to allow the formation of the above discussed short-circuit producing deposits in a practical time period. That is, for example, insufficient D.C. bias to form the shorting deposit in a period shorter than that required to use up the Grignard reagent starting material in a batch type operation or in a period approximately that between the normal maintenance shut-downs of a continuous process.

The process can be operated without significant buildup of the shorting deposit provided the D.C. component of the total current does not exceed about 10 to 15% of the total applied current. In any case, excessive D.C. bias can be readily detected by monitoring the current requirement during process operation. Rising amperage indicates deposit induced shorting.

Of course, the process must be operated Within an alternating frequency range defined by a frequency high enough to prevent formation at the electrodes of the shorting deposit during their anodic phase and at a frequency low enough to produce practical yields of the desired organometallic product. A technically very effective frequency range is from about 10 to about 1000 c.p.s. For reasons of economy and effectiveness one would operate generally with 50 to 60 c.p.s. current which is commercially available.

It Will be appreciated that the above alternating current process may comprise an interrupted ow of current first in one direction and then quantatively and oppositely in time, current and potential in the other direction. However, this reversal of current flow may be most economically and efficiently effected by utilizing ubiquitously available commercial alternating current electricity. In whatever way such current reversal is effected, the results are that the product i t (i.e., current times time) is practically equal in each direction.

Since, as discussed above, electrode-associated magnesium is continuously and rapidly dissolved by either the hydrocarbyl halide or the dihydrocarbyl sulfate, a clean (i.e. magnesium-free) metal surface is always presented to the electrolyte for the anodic oxidation of the electrode metal, which oxidation leads to organometallic compound formation.. A clean anode surface is highly desirable for efficient oxidation of the metal. This is especially true since magnesium metal is, electrolytically, more easily oxidized than many electrode metals. Any electrol lytic magnesium oxidation during the process would be at the expense of the desired oxidation of electrode metal.

The results of so conducting this alternating current process are as follows:

(1) An organometallic compound is continuously produced.

(2) No production shut-downs are required for occasional current reversal to keep an electrode free of short-circuit producing deposits. Electrode cleaning is continuous.

(3) The electrodes may be very closely spaced one to another without danger of shorting due to build-up of deposits. With such close electrode spacing, higher current densities may be employed than in direct current processes; thus, a high rate of organometallic compound production is attained without using high potentials to produce a practical flow of current from electrode to electrode. Production of excessive heat is, therefore, reduced or eliminated.

(4) The metal of the electrodes is uniformly consumed at a uniform rate. The process may, therefore, be operated with a single type of electrode as regards surface area, materials of electrode construction, weight, etc. Of course, dissimilar sacrificial metals, e.g. tin and lead may be utilized to prepare mixed organometallic compounds.

ALTERNATING CURRENT ELECT ROLYTIC CELLS Any suitable electrolytic cell may be utilized in this process. In laboratory cells a pair of solid, planar sacrificial metal electrodes may be arranged in laboratory glassware with the planar surfaces of the electrodes parallel. Mercury electrodes may take the form of electrically separated pools. The electrode surfaces are closely spaced but out of metal-to-metal contact, separated by the above described electrolyte and operably connected to a source of alternating current electricity. In larger equipment, a cell may be utilized which has a pair or a group of planar electrodes, curved surface electrodes, electrodes made up of rods or the like. Still other suitable electrodes could, alternatively, be made up of particulate metal held in a suitable metal retaining electrolyte-permeable, i.e., porous, inert container. Advantages of this latter type of electrode are that close electrode spacing may be maintained as electrode metal is consumed, and that electrode metal may be renewed as it is consumed simply by adding more particulate metal to the containers. Such renewable electrodes are, for example, described for the direct current process of Pearce et al. in U.S. Pat. No. 3,180,810.

The fundamental requirements for any such cell comprise the following:

(1) Means of operably electrically connecting electrodes to a source of alternating current electricity.

(2) Means of maintaining electrodes in close proximity to each other, but not in metal-to-metal contact. A solid electrode spacing of, for example, from about l mm. lo about 20 mm. (preferably 1 to 5 mm.) promotes cell efficiency, reduces the potential needed and, therefore, as discussed above, reduces the IR heating in the cell and, thus, minimizes cooling requirements.

(3) Means of adding the electrolyte to the cell and maintaining the electrolyte between electrodes.

(4) Means of removing spent electrolyte and product organometallic compounds from the cell.

(5) Means of circulating the electrolyte among the electrodes to prevent inefficiency due to localized electrolyte depletion and resultant poor conductivity.

(6) Means of controlling temperature in the electrolyte and in the electrodes.

(7) Means of keeping the water-reactive hydrocarbyl Grignard reagent substantially out of contact with gross Water or moisture.

It will be appreciated that a single 2-electrode cell, or that a battery of such 2-electrode cells, may be used. ln such a battery, electrical connections between cells may be series or parallel connections. A cell containing a plurality of separate electrodes may also be employed. A simple example of a multiple electrode cell would be a cell in which a group of electrodes is connected to one pole of an alternating current electricity source while another group of such electrodes is connected to the other pole of the source. Another type of multielectrode cell may be easily visualized by reference to the schematic diagram of the drawing. Output current and potential from three secondary windings (1, 2 and 3) of a three phase transformer are electrically connected via conductors 4, 5 and 6 to electrodes 7, 8 and 9. These electrodes are immersed in the electrolyte contained in cell 11. Such a system allows each electrode polarity to alternate with the alternating current, i.e. to become cathodic and then anodic to an equal but opposite degree with respect to time, potential and current. Since the three electrodes are 120 degrees out of phase in time from each other in current and potential, they become anodic and then cathodic, each electrode in turn, but not to the same degree at the same instant. For example, at any instant when electrode 7 has an anodic polarity and an organometallic compound is being produced at its surface, electrode 8 may be electrically neutral and, therefore, chemically inactive, while electrode 9 is cathodic and magnesium is being deposited and dissolved at its surface as described above. When electrode 7, 8 and 9 are symmetrically spaced with respect to one another, the above instantaneous situation varies continuously and symmetrically and each electrode in turn varies, in one cycle of alternating current electricity, from neutral (nil) current and neutral (nil) potential through maximum anodic potential and current and then back through neutral to maximum cathodic potential and current and finally, to complete the single cycle, back to neutral in current and potential.

The above cell descriptions are not meant to limit the scope of this invention. Many variations of the above and other cells will be apparent to those skilled in the arts of alternating current manipulation and electrolytic process technology.

It will be appreciated that the preferred electrode separation, about l mm. (millimeter) to about 5 mm. in a c ell, should not be taken as lirni-ting. Such separation distances are especially applicable for solid, planar or curved surface electrodes. With such electrodes thermal warping may produce metal-to-rnetal ishorting if distances much less than about 1 mm. tare used, while distances over 5 mm. are operable with solid electrodes, they are usually unnecessary. On the other hand, with particulate metal electrodes held in separate compartments or containers by an electrolyte permeable, metal retaining inert membrane, some of the particles in separate electrodes may be separated only by membrane thickness which can be substantially less than 1 mm. On the other hand, many of the particles in the separate containers may be substantially more than 5 mm. apart, inasmuch as a bed of particles in such a compartment must have some finite thickness and/or depth (c g., l0 to 20 mm.) to assure electrical contact between particles and to afford enough space between the membranes of a single compartment so that electrode renewal, by adding particulate metal, is not restricted. The key factor is obtaining minimum spacing consistent with avoiding Shorting.

In the following portions of this specification alternating current and alternating potential values will be given as the well known RMS values (i.e., root-mean-square values) of alternating current electricity. When alternating current and potential vary symmetrically, their variations may be plotted against time to obtain a symmetrical, sinusoidal curve about a nul or zero value line on the plot. With such sinusoidally varying current, the chemical equivalency of liampere (RMS) flowing for 1 hour is 33.6 meq. (milliequivalents) of electrochemical activity. Thus, an RMS ampere-hour produces theoretically 33.6 meq. of chemical change. In a process, then, 1 (RMS) amperehour producing 33.6 meq. of chemical change would afford a 10() mole-equivalent percent yield of said change.

In this invention there is utilized a cell such as one of the cells described above. If a tetrahydrocarbyl lead product is desired, the cell will have lead electrodes and, alternatively, electrodes of another metal when some other organometallic compound is desired. An electrolyte is circulated in the spaces Separating the electrodes by any suitable means of circulation, e.g., by internal agitation in the cell or by external pumping means which afford the desired circulation -of electrolyte. The electrolyte consists essentially of a hydrocarbyl `Grignard reagent (RMgX), which reagent is dissolved in a suitable inert organic solvent for the Grignard reagent. The solvent is preferably from about 0.5 molar to about 3 molar in Grignard reagent and also has dissolved therein from about 0.2 mole to about 2 moles of a compound R'X per mole of the Grignard reagent. R is a hydrocarbyl group with from 1 to about 12 carbons and X is a magnesium-displaceable group such that fRX can form a Grignard reagent, RMgX, or otherwise react with electrode associated magnesium and thereby dissolve said magnesium. It should be understood that Grignard reagent concentrations less than 0.5 molar are operable but sometimes impractical because of poor conductivity. At concentrations above about 3.0 molar, phase separation can occur. However, such phase separation is not necessarily detrimental in certain electrolytic cells. Thus the 3.0 molar limit may refer only to the amount of Grignard reagent actually in solution and not necessarily to the amount present in the whole system. The amount present in the whole system could be, for example, 2 to 10 times or more the amount necessary to provide a solution. Cell content temperature is adjusted to and held at between a low temperature defined by a suiciently low temperature to cause uneven, inefficient cell operation due to phase separation in the electrolyte or solidiiication of the electrolyte and an upper temperature defined by the thermal decomposition tempera-tures of the organometallic product or any of the electrolyte components. Next, a source of alternating current electricity, at an operable frequency, e.g. of from about 10 to 1000 cycles per second, is electrically connected to the electrodes so that a potential of from about 5 to about 85 volts provides a current density of from about 2 to about 20 ampere per dm.2 of electrode surface. Electrolysis may be continued until the Grignard reagent is substantially exhausted and the organometallic product recovered by any suitable means. Such means can be used as drowning the reaction mass in excess water and fractionally vacuum or fractionally steam distilling the water insoluble material. Of course, normally solid organometallic products such as tetraphenyl lead may be precipitated from the electrolyte by the addition of a solvent, in which such materials are insoluble, followed by filtration to recover the solid product. Alternatively, the process may be so conducted that Grignard reagent concentrations, coreagent concentrations, byproduct magnesium salt Concentrations and product concentrations in the electrolyte remain constant or vary over only a small range. Means of accomplishing constant concentration operation are known in the art. For example, portions of the electrolyte are periodically or continuously removed from an operating cell and appropriate quantities of fresh electrolyte are added back to the cell periodically or continuously. Organometallic compounds are recovered from the removed portions of the electrolyte by means of solvent extraction or vacuum fractional distillation. The magnesium salts are precipitated for subsequent removal by the addition of a suitable solvent which does not react with residual Grignard reagent or by the simple expedient of running the above distillation to the point where still bottom concentration afords salt precipitation. The residual Grignard reagent solution can then be either returned directly to the operating cell or so returned after standardization with more Grignard reagent, more coreagent, and, if necessary, more solvent. One such continuous or semi-continuous means of operation is disclosed, for example, by Braithwaite in U.S. Pat. No. 3,007,858.

It will be appreciated that a suitable inert organic solvent for a Grignard reagent comprises a substantially anhydrous ether such as diethylether, tetrahydrofuran, dialkyl ethers of diethylene glycol such as the dimethyl-, diethyl-, dipropyl, dibutyl-ethers of diethylene glycol and the like. Also included are mixtures of such ethers and normally liquid aromatic hydrocarbons such as benzene, toluene, xylene and the like. By inert is meant a solvent which remains substantially unreactive and unchanged by electrochemical and chemical reactions occurring in the cell. Suitable solvent combinations are disclosed, for example, by Linsk in U.S. Pat. No. 3,298,939. The Grignard reagents of this invention comprise alkyl, alkenyl or aryl Grignard reagents wherein the alkyl or alkenyl group has from 1 to about 8 carbon atoms and the aryl group from about 6 to 12 carbons. Useful Grignard reagents would be those wherein R is methyl, ethyl, npropyl, isopropyl, n-butyl-, sec-butyl, octyl, vinyl, allyl, phenyl-, toluyl, xylenyl, naphthyland the like.

Useful coreagents for the above Grignard reagents include methyl chloride, methyl bromide, methyl iodide, ethyl bromide, ethyl iodide, diethyl sulfate, propyl bromide, vinyl bromide, allyl chloride, allyl bromide, bromobenzene, bromotoluene, bromoxylene, bromonaphthalene and the like.

Since the process is preferably used to prepare tetramethyl lead, tetraethyl lead and mixed tetrahydrocarbyl leads, the preferred Grignard reagents are methylmagnesium chloride, methylmagnesium bromide, vinyl magnesium bromide and the corresponding ethylmagnesium compounds. As for the coreagent, the most preferred are methyl chloride, methyl bromide, ethyl bromide and diethylsulfate. Methyl and ethyl iodides, while operable, are usually not preferred because of poor solubility of the Grignard reagents formed by the action of the methylor ethyl-iodides with electrode deposited magnesium. The preferred concentration range for the Grignard reagent, about 0.5 to about 3 molar, is a practical range as described above. One mole of coreagent, RX, is required stoichiometrically for each mole of Grignard reagent to dissolve electrode associated magnesium; as little as about 0.2 mole is operable but not preferred because of the possibility of magnesium build up on the electrodes; and more than 2 moles are operable but serve no useful purpose. While the cell content temperature range defined above represents the extremes of operability, temperatures above 20 C. and below about 100 C. are practical because of cooling economy. In some of the alkyl lead syntheses, a minimum temperature of about 30 C. avoids formation of hexaalkyldileads. The alternating current frequency range, about 10 to about 1000 c.p.s., is, once again, a practical range. Frequencies below l c.p.s. are operable. Of course, at 0 c.p.s. one would have a direct current process and all its deficiencies. At frequencies greater than 1000 c.p.s. current efficiency, i.e., amount of desired hydrocarbyl metal product per ampere-hour, may drop to an impractical level. Frequencies of 50 to 60 c.p.s. are almost universally available commercially and very effective. Voltages less than about volts are operable but too low to maintain desired current densities, viz, 2 to 20 amperes per dm.2 of electrode surface. Voltages substantially greater than 85 volts, even voltages of several hundred RMS volts, are effective, but cell content heating can become excessive with high potentials and practical cooling difficult. Less than 2 amperes per dm.2 of electrode surface is technically efficient and operable, but can cause poor productivity with time. Above amperes per dm.2 current efficiency (i.e., ampere-hour yield) can become impractically low.

PREFERRED EMBODIMENTS In a preferred embodiment, tetramethyl lead is prepared at very high current densities, eg., up to 20 amperes per dm.2 and with very good current efficiency. A solution, in one of the solvents described above, about 0.8 to about 1.5 molar in methylmagnesium chloride and containing at least l mole of methyl chloride per mole of the Grignard reagent is electrolyzed at lead electrodes in a cell such as is described above. The preferred alternating current frequency is 60 c.p.s. Voltages from about 20 to 70 volts are preferred and a current density between about 10 and about 2O amperes per dm.2 of electrode surface is very effective. Temperatures between 30 and 60 C. are preferred. The process may be conducted substantially continuously as discussed above or batchwise. Batch-wise recovery of tetramethyl lead is feasible by any of several well known means, for example by fractional vacuum distillation of the electrolyte containing the tetramethyl lead.

It will also be appreciated that the low boiling methyl chloride, atmospheric pressure boiling point 24 C., has to be kept in the electrolyte by some means at a concentration providing about 1 mole of methyl chloride per mole of Grignard reagent. One means is to continuously bubble excess methyl chloride gas into the reaction mass. Unreacted methyl chloride may then be recovered by standard methyl chloride recovery procedures of tetramethyl lead technology. Alternatively, the process may be conducted under pressure sufficient to keep the desired quantity of methyl chloride in the electrolyte.

In another preferred embodiment, tetraethyl lead is produced. The process is conducted with 60 c.p.s. electricity substantially as in the preceding embodiment. In this case, however, the electrolyte solution is about 0.8 to 1.5 molar in ethylmagnesium chloride or ethylmagnesium bromide and about 0.8 to about 3 molar in diethyl sulfate. Current densities of from about 10 to about 20 amperes per dm.2 are obtained with impressed voltages of l0 to 40 volts. The process is conducted at about 30 to 60 C. Tetraethyl lead recovery is by one of the methods discussed above.

In a third preferred embodiment, mixed methylethyl leads, (CH3)yPb(C2H5)4 y, with y being a number of from 1 to 3, are produced. An electrolyte solution as described above, but from about 0.8 to about 1.5 molar in ethylmagnesium chloride and about 0.8 to 3 molar in methyl chloride is electrolyzed as above. Sixty c.p.s. and voltages of from about 20 to about 70 volts are employed to give 10 to 20 amperes per dm.2 of lead electrode surface. Temperatures preferred are between about 30 and about 60 C.

When the methylethyl lead product is isolated from the electrolyte substantially as soon as the methylethyl lead forms, substantially pure methyltriethyllead is the product. Means of isolation are as above discussed. When, on the other hand, electrolysis is conducted for a time suiciently long to afford substantial quantities of methylmagnesium chloride, from reaction of electrode deposited magnesium with the coreagent, methyl chloride, the process tends toward producing the structure Conversely, operating as immediately above with the electrolyte above 0.8 to about 1.5 molar in methylmagnesium chloride and about 0.8 to about 3 molar in ethyl bromide, the initial product tends to be trimethylethyl lead and, after longer electrolysis times, dimethyldiethyl lead.

EXAMPLES The following examples more fully illustrate the effectiveness and versatility of this invention. In these examples, electrochemical reactions and yields are calculated on a basis of 2-Faraday (i.e., 2r) per molecule of organometallic compound. Such an equivalency assignment is not completely arbitrary. That the process is possibly a 2- Faraday process or essentially so, is indicated, for example, by Linsks teaching in the Linsk patent cited above.

Example 1 To a magnetically agitatable, water-jacketed glass resin kettle of about 500 ml. (milliliters) capacity is added 200 ml. of a 1 molar solution of methylmagnesium chloride in tetrahydrofuran. Two solid planar lead electrodes attached to copper wire electrical leads are immersed in the Grignard reagent and fixed in position so that they are planareface parallel and about 4 mm. apart. The electrodes each have an immersed, opposing face area of 0.1 dm?. The electrode pair is placed near the periphery of the kettle to maximize solution circulation between electrodes as magnetic stirring produces a tangential ow of solution. Agitation is initiated, the temperature in the solution is adjusted to about 30 C. and methyl chloride gas bubbling into the solution is initiated using a sintered glass gas bubbler. The leads are connected to a source of 60 cycles per second alternating current electricity and 1 ampere (RMS) is passed through the solution between the electrodes under 59 volts1(-RMS) for 3.33 hours. Thus, 3.33 ampere-hours are passed at a current density of 10 amperes per dm.2 of electrode surface. Temperature in the solution is maintained at 30 to 31 C. during electrolysis. After the 3.33 hour electrolysis, the current source is disconnected and methyl chloride bubbling and agitation is stopped. The contents of the kettle are added to a large excess of water. The water suspension is extracted with several portions of diethyl ether. The combined ether extracts are stripped of diethyl ether and tetrahydrofuran by vacuum stripping. The dense liquid residue from the stripping is substantially pure tetramethyl lead as indicated by infrared spectroscopy and by a vapor phase chromatographic test using a previously calibrated vapor phase chromatograph. The yield is 69 mole percent of the theoretical yield based on the number of ampere-hours passed through the solution. Lead loss from the electrodes substantially corresponds stoichiometrically to the amount of tetramethyl lead produced. |Except for magnesium chloride recovered from the aqueous layer, there is substantially no other byproduct detected. After the electrolysis the opposing electrode faces are clean, bright and very evenly eroded. v

The following 4 examples illustrate making tetraethyl lead by the alternating current process.

[Example 2 In an experiment conducted in the apparatus as described in Example 1, an electrolyte consisting of a tetrahydrofuran solution 1 molar in ethylmagnesium bromide and 1 molar in diethyl sulfate is electrolyzed at 3l to 33 C. with 60 cycle alternating current at l2 volts (RMS). In a 2.04 ampere-hour run at 10 ampere (RMS) per dm.2 of electrode surface, an 88 mole percent yield of tetraethyl lead is obtained, based on ampere-hours. Tetraethyl lead is the sole organometallic product.

Example 3 A tetrahydrofuran solution 1 molar in ethylmagnesium bromide and 1 molar in ethyl bromide is subjected to alternating current electrolysis as in Example 2 at 60 cycles per second and at a temperature of from 50 to 60 C. After a 4.53 amperehour electrolysis at 15 amperes (RMS) per dm.2 and at 80 to 84 volts (RMS), tetraethyl lead yield was 58 mole percent based on amperehours passed.

Example 4 In an experiment conducted as in Example 2, tetrahydrofuran solution l molar in ethylmagnesium chloride and 1 molar in ethyl bromide is electrolyzed at 60 cycles per second and at 43 to 44 C. After a 2.5 ampere (RMS)- hour run at 42 volts (RMS) and at 10 amperes per dm.2, a 37% yield of tetraethyl lead is obtained, based on current passed.

Example 5 When the process is repeated substantially as in Example 4 with 1 molar ethyl chloride replacing the 1 molar ethyl bromide, tetraethyl lead yield is about 3 mole percent based on ampere hours.

The next example demonstrates the effect of coreagent absence.

Example 6 A 2.5 ampere-hour, 60 cycle electrolysis as in Example 2 with l molar ethylmagnesium bromide in tetrahydrofuran, at 49 C. at 10 amperes (RMS) per dm.2 and at 84 volts (RMS), failed to produce detectable quantities of tetraethyl lead or any other organometallic compound derived from the lead electrodes.

The following observation may be made about the preceeding 6 examples of this alternating current process:

(l) The alkyl Grignard reagent-coreagent couple, viz, IRMgX-RX, is not particularly sensitive, as regards -good yield results, to the nature of X in RMgX.

(2) When the coreagent is methyl chloride good yield results are obtained, but, surprisingly, ethyl chloride is an operable but very poor coreagent.

(3) A dialkyl sulfate coreagent provides surprisingly superior results in terms of yield and high current density with relatively low RMS voltage.

(4) A coreagent is, very surprisingly, required for process operability. As is shown in Example 6, no tetrahydrocarbyl metal is produced in the absence of a Coreagent.

The following example illustrates the use of the 60 cycle alternating current process for preparing mixed tetramethylethyl leads.

Example 7 Two separate experiments are conducted utilizing the equipment described in Example 1 to electrolyze a solution containing about 25 vol. percent of the dimethyl ether of diethyldiethylene glycol and 75 vol. percent tetrahydrofuran, which solution is about 1 molar in ethylmagnesium chloride and about 1 molar in methyl chloride. At temperatures between 31 and 45 C. and RMS voltages from 23 to 27 volts and with a l ampere (RMS)- hour run, substantially pure methyltriethyl lead is obtained in 72 mole percent yield based on 1 ampere-hour passing. When, however, in a separate operation a 6- ampere-hour run is employed with about the same current density a 66 mole percent yield of a mixture having the average formula, (CH3)l.7Pb(C2H5)2.3 is obtained. Product structure is determined by means of vapor phase chromatography.

Thus with longer-time electrolyses there is a tendency towards the average structure (CH3)2Pb(C2H5)2. This same tendency to a dimethyldiethyl lead average structure with longer electrolysis is apparent when a tetrahydrofuran solution about 1 molar in methylmagnesium chloride and about 1 molar in ethyl bromide is electrolyzed. A short ampere-hour electrolysis tends to produce trimethylethyl lead, while a longer ampere-hour run tends to produce the product having an average formula represented by (CH3)2Pb(C2H5)2.

The following example demonstrates the effect of changing the alternating current frequency on product yield.

Example 8 A series of separate experiments are conducted as in Example 2 under the following standardized conditions and with the following reagent quantities.

1 molar each in 200 ml. of tetraf CzHsMgBr:

hydroturan solution 1 l Varying the frequency of the alternating current produced the effect shown in the following table.

Frequency, cycles Tetraethyllead yield,

per sec.: mole percent1 (direct current)2 80 20 86 60 83 500 67 1000 48 5000 3 1Based 0n ampere-hours. 2 Not RMS voltage or amperage.

Quite surprisingly, yields at 20 and 60 c.p.s. are superior to a direct current yield wherein the same amount of electrolyzing current is passed through the electrolyte.

Example 9 Example 1 is repeated using 0.1 drn.2 tin electrodes and 60 cycle current at a temperature of 32 to 34 C. With 200 ml. of an electrolyte 1.5 molar in methylmagnesium chloride in a 9 ampere-hour run at 15 amperes/dm.2, a 50 mole percent yield of tetramethyl tin is obtained. Product identification was by boiling point, infrared and vapor phase chromatographic comparisons with a known sample of tetramethyl tin.

Example 10 In an experiment conducted substantially as in Example 2, 0.1 dm.2 bismuth electrodes are employed. A tetrahydrofuran solution l molar in ethylmagnesium chloride and 1 molar in ethyl bromide is electrolyzed at about 35 C. and at 10 amperes per dm?. After about 1.75 hours of electrolysis, i.e., 1.75 ampere-hours, the yield based on ampere-hours is 70 mole percent assuming a triethyl bismuth product. Substantial decomposition of the triethyl bismuth is apparent.

Example l1 An agitatable, jacketed electrolytic cell is prepared with two separated pools of mercury in the bottom of the cell. The pools are in the form of concentric rings of about 0.12 dm.2 surface area each. Electrical connection between the pools and a 60 cycle per second alternating current source is provided by platinum wire leads which, within the cell, were in electrical contact with the mercury but not with the electrolyte. A 200 rnl. solution of 1 molar ethylmagnesium bromide and l molar ethyl bromide in tetrahydrofuran is electrolyzed at 0.6 ampere (RMS) and at about 118 to about 140 volts (RMS) for hours. A diethyl mercury yield of 16 mole percent is obtained.

Example 12 A tetrahydrofuran solution 0.9 molar in phenylmagnesium bromide and 1.1 molar in bromobenzene is electrolyzed at 60 cycles per second in the cell described in Example 1. At temperatures which varied from 30` to 45 C. during the run, a 2.0 ampere (RMS)-hour run at amperes/dm.2 produced tetraphenyl lead which was identified by means of the mixture melting point technique. During the run, voltage fell steadily from 84 volts (RMS) to 30 volts (RMS), PbB2 was produced along with the tetraphenyl lead. Tetraphenyl lead yield was less than 50 mole percent based on ampere-hours.

Example 13 The experiment of Example l is repeated utilizing 400 ml. of a tetrahydrofuran solution about 0.7 molar in vinylmagnesium bromide. About 2.5 ampere-hours (RMS) are passed through the cell in 2.8 hours at between about 27 and about 46 C. The product mixture, lsolated as in Example l, is found by vapor phase chromatography to be a mixture comprising tetravinyl lead, trivinylmethyl lead, divinyldimethyl lead, vinyltrimethyl lead and tetramethyl lead.

As seen from the above examples, the subject invention provides substantial advantages including:

(l) An improved electrolytic process for preparing hydrocarbyl metal compounds in which process deposit induced shorting of sacrificial metal electrodes is overcome;

(2) A process wherein the electrodes are composed of the same metal and, during electrolysis electrodes alternate between being anodic for half the time, and then quantitatively and oppositely cathodic for the same time period;

(3) A process wherein the above alternation in electrode polarity is obtained by operably connecting electrodes to a source of alternating current electricity; and

(4) A process which does not require rectification of available A.C. electricity.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for obvious modifications will occur to those skilled in the art.

What is claimed is:

1. A process for preparing organometallic compounds of the formula RyR yM where R and R' are separately selected from alkyl and alkenyl groups having from l to about 8 carbon atoms and aryl groups having from 6 to about 12 carbon atoms and wherein R and R may be the same or different, M is a metal selected from the metals of Groups II-B, IV-A and V-A of the Periodic Table, n is a number corresponding to the valence of M and y is a number from 0 to 4, which process comprises (A) passing an electrolyzing practically symmetrical alternating electric current through an electrolyte solution comprising a hydrocarbyl Grignard reagent RMgX, wherein R is as defined above and X is a halide other than fluoride, in an inert organic solvent for said reagent between electrodes consisting essentially of a metal M as defined above, said solution also containing from about 0.2 to about 2.0 moles per mole of RMgX of a compound of the formula RX' wherein R is as defined above and X is selected from a halide other than fluoride and a lower alkylsulfate anion; and

(B) recovering the resultant RyR' yM.

2. Claim 1 wherein said electrolyte solution is kept under agitation in step A and wherein the concentration of said Grignard reagent is about 0.5-3.0 molar.

3. Claim 2 wherein said electrolyzing alternating electric current has a frequency in the range of l0 to 1000 cycles per second and a potential sufiicient to provide a current density of from about 2 to 20 amperes per dm.2 of electrode surface.

4. Claim 3 wherein Step A is performed at a temperature in the range of about 20-100 C.

5. Claim 4 wherein said electrodes are spaced from about 1 to about 20 mm. apart.

6. Claim 2 wherein M is selected from lead, tin bismuth and mercury.

7. Claim 6 wherein M is lead and n is 4.

8. Claim 7 wherein R and R are methyl, and X and X are chloride.

9. Claim 7 wherein R and R are ethyl, X is selected from chloride and bromide and X is selected from bromide and ethylsulfate.

10. Claim 9 wherein X is bromide and X' is ethylsulfate.

11. Claim 7 wherein R is ethyl, R is methyl, X and X are chloride and y is a number from 1 to 3.

12. Claim 11 wherein the resultant (C2H5)yPb (CH3)4 y is continuously removed from the electrolyte substantially as soon as it is formed to obtain substantially pure methyltriethyl lead.

13. Claim 7 wherein R is methyl R is ethyl, X is chloride, X is bromide and y is a number from 1 to 3.

14. Claim 13 wherein the resultant (CH3)yl"b(C2H5).1 y is continuously removed from the electrolyte substantially as soon as it is formed to obtain substantially pure trimethylethyl lead.

15. Claim 7 wherein R and R are phenyl, and X and X are bromide.

16. Claim 7 wherein R is vinyl, R is methyl, X is bromide and X is chloride.

17. A process for preparing organometallic compounds of the formula RyR4 yPb, wherein R and R are separately selected from methyl, ethyl, phenyl and vinyl wherein R and R' may be the same or different and y is a 10 number from 0` to 4, which comprises:

(A) passing a practically symmetrical alternating current of 20-100 c.p\.s. and 10-70 volts through an agitated electrolyte solution between electrodes consisting essentially of lead, said electrolyte solution comprising a hydrocarbyl Grignard reagent RMgX, wherein R is as defined above and X is selected from chloride and bromide, in an inert organic solvent for said reagent, the concentration of said reagent being -from 0.5 to 3.0 molar and said solution also containing from 0.2 to 2.0 moles per mole of RMgX of a compound of the formula R'X' wherein R' is as deiined above and X is selected from chloride, bromide and ethylsulfate, at a temperature of 20-60 C., and

(B) recovering the resultant RyR4 yPb. 18. Claim 17 wherein said solvent is tetrahydrofuran.

References Cited UNITED STATES PATENTS 2/ 1966 Braithwaite 204-59 15 JOHN H. MACK, Primary Examiner N. A. KAPLAN, Assistant Examiner 

